Cross-linkable adhesive polymers

ABSTRACT

An adhesive composition formed by admixing under melt conditions (a) 50-95% by weight of a moisture cross-linkable silane-grafted polyolefin having a melt viscosity of less than 100,000 cps, and (b) 5-50% by weight of a polyolefin grafted with at least one ethylenically-unsaturated carboxylic acid or anhydride, and derivatives thereof. The grafted polyolefin has a melt viscosity of less than 100,000 cps, and contains a catalyst for moisture cross-linking of the silane-grafted polyolefin of (a). The adhesive may be used to bond a variety of substrates.

CROSS-LINKABLE ADHESIVE POLYMERS

This is a division of application Ser. No. 08/237,803, filed May 4, 1994U.S. Pat. No. 5,461,110.

FIELD OF THE INVENTION

The present invention relates to a cross-linkable adhesive polymercomposition which is capable of being eapplied to a substrate in amolten form and then cross-linked subsequent to application to asubstrate.

BACKGROUND

Structures may be bonded together using a variety of adhesives. Oneadhesive that is relatively easy to apply while still providing goodbonding properties is an isocyanate-containing polyurethane adhesive,which may be sprayed onto a substrate and then subsequently cured toprovide a strong bond. However, the use of such polyurethane adhesivesin lamination processes represents an occupational health risk and majorprecautions must be taken during operation of the process in order toprotect the personnel involved. An alternate adhesive under developmentis a moisture curable 100% solid hot-melt polyurethane adhesive, but itis understood that such adhesives developed to date suffer fromstability problems and may not have the desired bond strength for manyend uses. It would be desirable to have alternate processes that do notuse polyurethane adhesives.

The cross-linking of a polymer is used to alter the properties of thepolymer. Thus, it is possible to fabricate a polymer into a shape,including in the form of an adhesive, and then to cross-link the polymerto improve the end use properties. In one such method, vinyl silanes,e.g., vinyl trimethoxysilane (VTMS) and vinyl triethoxysilane (VTES),are grafted onto polymers. The grafted polymers are formed into adesired shape under molten conditions and the resultant article issubsequently subjected to moisture, e.g., steam or atmospheric moisture,whereupon the silane residue forms cross-links between polymer chains.The grafting reaction of the vinyl silane onto the polymer is usuallycarried out in the presence of an organic peroxide, and thecross-linking reaction with moisture is usually carried out in thepresence of a cross-linking catalyst.

An adhesive for heat-sealing of metals and plastics is disclosed inJapanese Patent 1,161,079 of Showa Electric Wire KK, which issued Jun.23, 1989. The adhesive composition disclosed is composed of 100 parts byweight of one or more of polyethylene, polypropylene, ethylene/vinylacetate copolymer or ethylene/ethyl acrylate copolymer grafted withmaleic acid and/or acrylic acid in amounts of 0.5-10 wt. %, and 0.1-5parts by weight of a silane coupling agent. Preferably the adhesivecomposition is blended with 0.05-3 wt. % of an organic peroxide that hasa half-life of one minute at a temperature in the range of 100°-180° C.The adhesive is stated to have good adhesion and heat resistance in thebonding of plastics to metal and plastics to plastics, but would not bestable under melt conditions and hence not useful as a hot meltadhesive.

As used herein, melt viscosity is measured at 190° C. on a Brookfieldviscometer at a shear rate of 0.35 sec^(-l).

DESCRIPTION OF INVENTION

An adhesive with both storage stability and good bonding properties hasnow been found.

Accordingly, the present invention provides an adhesive compositionformed by admixing under melt conditions:

(a) 50-95% by weight of a moisture cross-linkable silane-graftedpolyolefin having a melt viscosity of less than 100,000 cps, and

(b) 5-50% by weight of a polyolefin grafted with at least oneethylenically-unsaturated carboxylic acid or anhydride, and derivativesthereof, said grafted polyolefin having a melt viscosity of less than100,000 cps, said grafted polyolefin containing a catalyst for moisturecross-linking of the silane-grafted polyolefin of (a).

In a preferred embodiment of the adhesive of the present invention, themoisture cross-linking catalyst is dibutyl tin dilaurate.

In a further embodiment of the present invention, the polyolefin isselected from ethylene/vinyl acetate copolymer, polypropylene,polyethylene, ethylene/alkyl (meth) acrylate and ethylene/(meth) acrylicacid copolymers and copolymers of ethylene, alkyl acrylate and carbonmonoxide.

The present invention also provides a process for forming an adhesivecomposition comprising:

(i) admixing under melt conditions

(a) 50-95% by weight of a moisture cross-linkable silane-graftedpolyolefin having a melt viscosity of less than 100,000 cps, and

(b) 5-50% by weight of a polyolefin grafted with at least oneethylenically-unsaturated carboxylic acid or anhydride, and derivativesthereof, said grafted polyolefin having a melt viscosity of less than100,000 cps, said grafted polyolefin containing a catalyst for moisturecross-linking of the silane-grafted polyolefin of (a), and

(ii) applying the molten adhesive so formed to a substrate.

The present invention further provides a process for the bonding of afirst substrate to a second substrate comprising coating the firstsubstrate with a composition formed by the admixing under moltenconditions of:

(a) 50-95% by weight of a moisture cross-linkable silane-graftedpolyolefin having a melt viscosity of less than 100,000 cps, and

(b) 5-50% by weight of a polyolefin grafted with at least oneethylenically-unsaturated carboxylic acid or anhydride, and derivativesthereof, said grafted polyolefin having a melt viscosity of less than100,000 cps, said grafted polyolefin containing a catalyst for moisturecross-linking of the silane-grafted polyolefin of (a), contacting thesecond substrate with the molten adhesive, and subsequently exposing thesubstrates and interposed layer of adhesive to moisture.

In embodiments of the processes of the invention, the molten adhesive isapplied to the first substrate within five minutes of the admixing undermelt conditions, preferably within one minute and especially within 30seconds.

The present invention also provides a bonded structure formed from twosubstrates and an interposed layer of an adhesive composition formed byadmixing under melt conditions:

(a) 50-95% by weight of a moisture cross-linkable silane-graftedpolyolefin having a melt viscosity of less than 100,000 cps, and

(b) 5-50% by weight of a polyolefin grafted with at least oneethylenically-unsaturated carboxylic acid or anhydride, and derivativesthereof, said grafted polyolefin having a melt viscosity of less than100,000 cps, said grafted polyolefin containing a catalyst for moisturecross-linking of the silane-grafted polyolefin of (a), said interposedlayer having been exposed to moisture to effect said cross-linking afterapplication of the adhesive between the two substrates.

In embodiments, the substrate is selected from polyolefins, cellulosicmaterials, e.g., wood, polyvinyl chloride, ethylene/propylene/dienecopolymers, chlorosulphonated polyolefins, nylon and metals.

A wide variety of polyolefins may be used in the adhesive of the presentinvention. Different polyolefins may be used for the silane-graftedcomponent and for the ethylenically unsaturated carboxylic acid oranhydride-grafted component. If different polyolefins are used, thenthose polyolefins, particularly in their grafted forms, must becompatible in order that the resultant adhesive has acceptableproperties. In preferred embodiments of the present invention, thepolyolefins used in the two components are of the same type, e.g., theyare both polypropylene, and in particularly preferred embodiments thepolyolefins have the same density and melt index.

The polyolefin may be a homopolymer of ethylene or a copolymer ofethylene with at least one C_(4-C) ₁₀ hydrocarbon alpha-olefin,especially copolymers of ethylene with butene-1, hexene-1 and/oroctene-1. Such polyolefins may be made by a wide variety of olefinpolymerization processes known in the art, including the high pressurepolymerization of ethylene, the formation of homopolymers and copolymersunder slurry conditions, in solution polymerization processes or ingas-phase polymerization processes. The so-called single-site ormetallocene catalyst processes may also be used to obtain the polymer.The density of the polymer may range from about 0.860 g/cm³ to about0.970 g/cm³, i.e., the full range known for such polymers. The meltviscosity, measured as defined above, is less than 100,000 cps.

The polyolefin may also be polypropylene. As used herein, the expression"polypropylene" refers to homopolymers of propylene, to impact orso-called block copolymers of propylene with ethylene in which theethylene content is less than about 25% by weight and to randomcopolymers of propylene with ethylene in which the ethylene content isless than 8% by weight. The polypropylene has the same melt viscositycharacteristics as the polyethylene described above.

Alternatively, the polyolefin may be a copolymer of ethylene and atleast one comonomer selected from carbon monoxide, vinyl acetate, alkylacrylates and alkyl methacrylates, in which the alkyl group has 1-4carbon atoms. In embodiments, the copolymer is characterized by having aheat of crystallization of less than 70 J/g. Moreover, if the copolymeris an ethylene/vinyl acetate copolymer, then the copolymer has a vinylacetate content of at least 10% by weight. Examples of the copolymersare ethylene/vinyl acetate copolymers, ethylene/methyl acrylatecopolymers, ethylene/ethyl acrylate copolymers, ethylene/butyl acrylatecopolymers, ethylene/isobutyl acrylate copolymers, ethylene/vinylacetate/carbon monoxide copolymers, ethylene/ethyl acrylate/carbonmonoxide copolymers, ethylene/butyl acrylate/carbon monoxide copolymers,ethylene/ethyl methacrylate/carbon monoxide copolymers andethylene/butyl methacrylate/carbon monoxide copolymers. The meltviscosity of such copolymers is the same as that described above forpolyethylene.

The vinyl silane is a vinyl trialkoxysilane selected from the groupconsisting of vinyl trimethoxysilane and vinyl triethoxysilane. Suchsilanes are available commercially. While compositions containing vinylsilane and grafting catalyst may be used, such compositions do notcontain a cross-linking catalyst.

The vinyl silane must be maintained in a moisture-free environment atall times prior to the desired time of cross-linking i.e., afterextrusion of the adhesive onto and between the substrates that are to bebonded, as will be understood by persons skilled in the art ofmoisture-curable adhesives. The vinyl silane is grafted onto the polymerused in component (a) of the adhesive composition. It is preferred thata grafting catalyst be used, e.g., an organic peroxide for exampledicumyl peroxide, in amounts of, for example, 0.01-0.25% by weight ofthe polyolefin. The concentration of the vinyl silane, the melttemperature and the residence time of the admixture under meltconditions during the grafting of the vinyl silane onto the polyolefinare all believed to be important variables in the grafting reaction. Thetemperature should be below the temperature at which the polyolefin issignificantly degraded. Liquid compositions of vinyl silane and graftingcatalyst are commercially available.

The polyolefin of component (b) is grafted with an ethylenicallyunsaturated carboxylic acid or anhydride. The grafting monomer is atleast one monomer selected from ethylenically unsaturated carboxylicacids and ethylenically unsaturated carboxylic acid anhydrides,including, less preferably, derivatives of such acids, and mixturesthereof. Examples of the acids and anhydrides, which may be mono-, di-or polycarboxylic acids, are acrylic acid, methacrylic acid, maleicacid, fumaric acid, itaconic acid, crotonic acid, itaconic anhydride,maleic anhydride, and substituted maleic anhydride, e.g., dimethylmaleic anhydride or citraconic anhydride, nadic anhydride, nadic methylanhydride, and tetrahydrophthalic anhydride, maleic anhydride beingparticularly preferred. Examples of the derivatives of the unsaturatedacids are salts, amides, imides and esters, e.g., mono- and disodiummaleate, acrylamide, maleimide, glycidyl methacrylate and dimethylfumarate. Techniques for the grafting of such monomers onto thepolyolefin are known, e.g., as described in U.S. Pat. No. 4,612,155 ofR. A. Zelonka and C. S. Wong, which issued Sep. 16, 1986, and inpublished European patent application No. 0,369,604 of D. J. Mitchell,published May 23, 1990. The present invention will be particularlydescribed herein with reference to maleic anhydride as the graftingmonomer.

The melt viscosity of the polyolefin composition should be less thanabout 100,000 cps, and preferably less than 50,000 cps. It is also knownto use blends of grafted and ungrafted polyolefins in order to achieve adesired level of graft in a composition.

A cross-linking catalyst for the moisture curing reaction isincorporated into component (b). Examples of cross-linking catalysts aretitanates and carboxylic acids salts of zinc, iron and tin, e.g., zincoctoate, tin octoate and dibutyltin dilaurate. The amount ofcross-linking catalyst used may be varied over a wide range as will beunderstood by those skilled in the art. In embodiments, dibutyltindilaurate may be used in amounts of at least 0.01% by weight of thepolymer to be cross-linked, especially 0.01-0.1% by weight, and inparticular about 0.05% by weight. Increasing the amount of cross-linkingcatalyst tends to increase the rate of cross-linking of the polymer butthe quality of the molten polymer may be adversely affected.

The adhesive is maintained in the form of two separate components untiljust prior to use. It is understood that both components would normallybe preformed i.e., having been formed in separate reactions, although itcould be envisaged that one or both components could be formed in situin separate reactions and then fed directly into the means used to admixand form the adhesive of the invention. It is also understood that themoisture cross-linkable silane-grafted polyolefin would be maintained ina moisture-free condition prior to forming of the adhesive, as is knownfor moisture-cross-linkable adhesives.

The adhesive of the present invention is formed by admixing components(a) and (b) under melt conditions. This may be achieved by admixing theadhesive in a pelletized form and subsequently extruding under meltconditions whereby the components are admixed to form the adhesive. Theadhesive is extruded directly onto a substrate i.e., while the adhesiveis still in a molten condition. Alternatively each of component (a) andcomponent (b) could be heated into a molten condition and the moltencomponents then admixed and extruded. Apparatus suitable for the mixingof the two components and application as an adhesive are known, e.g.,the two-component hot-melt adhesive applicator known as a Nordsontwo-component applicator system.

It is an important aspect of the present invention that the adhesivecomposition be applied to the substrates shortly after formation fromthe two components thereof. For example, two-component adhesiveapplicators may be used. In such applicators the adhesive might remainin an admixed form for a short period of time, e.g., only a few seconds.However, as illustrated hereinafter, the adhesive has poor stability inan admixed molten form, and thus should be used shortly afterpreparation, especially within five minutes of admixing under moltenconditions, preferably within one minute and especially within 30seconds.

The adhesive is applied to a first substrate in a molten condition andthen the second substrate is applied over the adhesive while theadhesive is still in a molten condition. Contact of the adhesive whilemolten with both substrates is important in order to achieve a goodbond. The resultant substrates with interposed layer of adhesive is thenexposed to moisture for a period of time until the bond between theadhesive and each substrate has developed to a sufficient degree. Thismay be done by simply exposing the bonded substrates to moisture in theatmosphere. Alternatively, the bonded substrates may be subjected tohigh humidity conditions, e.g., steam, in order to promote the adhesion.

Examples of substrates that may be bonded are polyolefins, cellulosicmaterials, e.g., wood, polyvinyl chloride, ethylene/propylene/dienecopolymers, chlorosulfonated copolymers, nylon and metals like aluminumand steel, and the like.

As used herein, the shear adhesion fail test is conducted according tothe procedure of ASTM D4498 "Heat-Fail Temperature in Shear of Hot-meltAdhesives".

The adhesive of the present invention may be used in the bonding of awide variety of substrates. In addition the adhesive is maintained in astable condition, as two separate components, until such time as it isneeded. The adhesive is extrudable with relative ease due to its highmelt index but it is subsequently cross-linked to form a strong adhesivebond.

The present invention is illustrated by the following examples.

EXAMPLE I

Component A was formed by grafting an ethylene/vinyl acetate copolymer,containing 18% vinyl acetate and having a melt index of 500 dg/minbefore grafting, with a mixture of vinyl trimethoxysilane and2,5-dimethyl-2,5-di-(t-butyl peroxy) hexane. The melt index of thepolymer after grafting was approximately 250 dg/min.

Component B was a maleic anhydride-grafted ethylene/vinyl acetatecopolymer containing 28% by weight of vinyl acetate and 0.8% by weightof grafted maleic anhydride. To this grafted polymer was added 5000 ppmof dibutyl tin dilaurate.

The melt viscosities of components A and B were measured over a periodof time at 190° C. in a Brookfield apparatus having a Thermoselattachment using a shear rate of 0.35 sec-1.

In addition, a blend of 90% by weight of component A and 10% by weightof component B was formed in a plasticorder at 150° C. for 5 minutes.The melt viscosity of the blend was also measured over a period of time.

The results obtained were as follows:

    ______________________________________                                                       VISCOSITY  VISCOSITY                                                          cps        cps                                                 COMPONENT      (15 min.)  (2 hrs.)                                            ______________________________________                                        A              27,600     32,000                                              B              22,000     30,000                                              Blend          >2,000,000 --                                                  ______________________________________                                    

These results show that components A and B would be suitable for use ina two-component hot melt adhesive applicator, because the melt viscositywas relatively stable over a period of two hours, showing only a smallincrease. In contrast the blend, which had been subjected to 150° C. for5 minutes prior to the viscosity test at 190° C. had a melt viscosityafter 15 minutes at 190° C. that would be unacceptable for use in atwo-component hot melt adhesive applicator.

EXAMPLE II

Component C was formed by grafting an ethylene/vinyl acetate copolymer,containing 28% vinyl acetate and having a melt index of 800 dg/minbefore grafting, with a mixture of vinyl trimethoxysilane and2,5-dimehtyl-2,5-di-(t-butyl peroxy) hexane. The melt index of thepolymer after grafting was approximately 400 dg/min.

Component D was a maleic anhydride-grafted ethylene/vinyl acetatecopolymer containing 28% by weight of vinyl acetate and 0.8% by weightof grafted maleic anhydride. To this grafted polymer was added 5000 ppmof dibutyl tin dilaurate.

The melt viscosities of components C and D were measured over a periodof time at 190° C. using the procedure of Example I.

In addition, a blend of 90% by weight of component C and 10% by weightof component D was formed in a plasticorder at 150° C. for 5 minutes.The melt viscosity of the blend was also measured over a period of time.

The results obtained were as follows:

    ______________________________________                                                       VISCOSITY  VISCOSITY                                                          cps        cps                                                 COMPONENT      (15 min.)  (2 hrs.)                                            ______________________________________                                        C              24,000     28,000                                              D              22,000     30,000                                              Blend          >2,000,000 --                                                  ______________________________________                                    

As in Example I, the results show that components C and D would besuitable for use in a two-component hot melt adhesive applicator, butthe blend would not be.

EXAMPLE III

To further illustrate the present invention, tests were conducted oncomponent A of Example I and on a 90/10 mixture of components A and B ofExample I. The mixture was formed in a Brabender mixer for 5 minutes at150° C., and then immediately applied to the substrate.

The following tests were done:

Tensile strength by the procedure of ASTM D638; Elongation at failure bythe procedure of ASTM D638;

Young's modulus by the procedure of ASTM D638; Shear adhesion failtemperature for aluminum to aluminum bonds, as described above;

Lap shear for an aluminum to aluminum bond using the procedure of ASTMD1002; and Gel content using the procedure of ASTM D2765.

The results obtained were as follows:

    ______________________________________                                        TEST          COMPONENT A   ADHESIVE                                          ______________________________________                                        Tensile Strengths                                                                           450           780                                               (PSI)                                                                         Elongation and                                                                              600           600                                               Failure (%)                                                                   Young's Modulus                                                                             1150          1200                                              (PSI)                                                                         Shear Adhesion                                                                              110           >199                                              Fail Temperature                                                              (°C.)                                                                  Lap Shear (PSI)                                                                             550           800                                               Gel Content (%)                                                                             --            44                                                ______________________________________                                    

The results showed that the adhesive had substantially higher shearadhesion fail temperature and lap shear in aluminum to aluminum bonds.

EXAMPLE IV

Using the procedures of Example III, tests were conducted on component Cof Example II and on a 90/10 mixture of components C and D of ExampleII, the latter having been formed in a Brabender mixer for 5 minutes at150° C., and then immediately applied to the substrate.

The results obtained were as follows:

    ______________________________________                                        TEST          COMPONENT C   ADHESIVE                                          ______________________________________                                        Tensile Strengths                                                                           280           468                                               (PSI)                                                                         Elongation and                                                                              540           600                                               Failure (%)                                                                   Young's Modulus                                                                             450           800                                               (PSI)                                                                         Shear Adhesion                                                                              82            >199                                              Fail Temperature                                                              (°C.)                                                                  Lap Shear (PSI)                                                                             460           760                                               Gel Content (%)                                                                             --            38                                                ______________________________________                                    

The results showed that the adhesive had substantially higher shearadhesion fail temperature and lap shear in aluminum to aluminum bonds.

Example V

An adhesive was formed from a 90/10 blend of components A and B ofExample I. The adhesive was formed in a Brabender mixer, for 5 minutesat 150° C., and then immediately applied to a substrate formed from achlorosulphonated polymer. A second substrate of the chlorosulphonatedpolymer was applied and subsequently the resultant structure wassubjected to moisture in the form of steam to cure the adhesive.

The structure was subjected to the lap shear test and to the shearadhesion fail test both before and after the moisture curing of theadhesive. It was found that curing of the adhesive increased the lapshear from 21 psi to 39 psi on curing and that the shear adhesion failtest value increased from 100° C. to 150° C. on curing.

The procedure was repeated using an ethylene/propylene/diene copolymer(EPDM) substrate instead of the chlorosulphonated polymer. It was foundthat the lap shear was unchanged on curing but that the shear adhesionfail temperature increased from 100° C. to 50° C. on curing.

The results show the value of the curable adhesives of the invention.

We claim:
 1. A bonded structure formed from two substrates and aninterposed layer of an adhesive composition formed by admixing undermelt conditions:(a) 50-95% by weight of a moisture cross-linkablesilane-grafted polyolefin having a melt viscosity of less than 100,000cps, and (b) 5-50% by weight of a polyolefin grafted with at least oneethylenically-unsaturated carboxylic acid or anhydride, or salt, amide,imide, or ester derivative thereof, said grafted polyolefin having amelt viscosity of less than 100,000 cps, said grafted polyolefincontaining a catalyst for moisture cross-linking of the silane-graftedpolyolefin of (a), said interposed layer having been exposed to moistureto effect said cross-linking after application of the adhesive betweenthe two substrates.
 2. The bonded structure of claim 1 in which thesubstrate is selected from polyolefins, cellulosic materials, polyvinylchloride, ethylene/propylene/diene copolymers, chlorosulphonatedpolyolefins, and nylon and metals.
 3. The bonded structure of claim 2 inwhich the polyolefin is selected from ethylene/vinyl acetate copolymer,polypropylene, polyethylene, ethylene/alkyl (meth) acrylate andethylene/(meth) acrylic acid copolymers and copolymers of ethylene,alkyl acrylate and carbon monoxide.